Data communication systems are known for the conveyance of digital information over a communication path such as a microwave of cable path. Digital information is generally provided to the system at a predetermined data rate, and the system is synchronized to and operative only at the selected rate to transmit the digital signal over the communication path and to recover the digital data at the receiving end. In order to provide synchronization with a particular data rate, appropriate clock or timing apparatus must be provided. In addition, synchronous encoding apparatus must often be provided to translate an input signal to a form suitable for transmission, with similar decoding apparatus provided for translating a received data signal to an intended output format. In known systems operative only at a specified clock rate, a change in the data rate would necessitate physical modification of the apparatus to be operative at the different rate.
A digital communication system capable of operation over a wide range of data rates without system modification is the subject of a copending application Ser. No. 545,790 in the names of Harvey L. Pastan and Arthur H. Solomon, filed of even date herewith and the disclosure of which is incorporated herein by reference. The present invention relates to a phase modulator especially adapted for use in the system of this copending application, although the invention is not limited to such use.
Phase modulators operative at microwave and other frequencies are well known in the art but in general are limited to use at a single carrier frequency or frequency band. In order to accommodate different carrier frequencies, the modulator structure must usually be modified to account for changes in impedance and wavelength at the intended operating frequency. One known modulator is shown in U.S. Pat. No. 3,506,930 wherein a phase shifting network is employed having a plurality of diodes and capacitors spaced along a transmission line. The diodes are selectively rendered conductive and non-conductive in response to control signals derived from the conductive diodes providing short circuits for a modulation signal, reflection of the carrier wave and consequent phase modulation thereof. The disclosure of the aforementioned patent contemplates the use of ideal diode switches which are either substantially conductive or non-conductive and which are not readily realizable in practice, especially at microwave frequencies.